Current state of knowledge
In order to develop essential advances in the field of nanofriction, and of tribology in general, fundamental research lines revolving around four critical topics are recognized as the most pressing for the current state of knowledge.
- Bridging tribological mechanisms at different scales. Friction takes place in different phenomena that span a wide spectrum of length scales, from sub-nanometre up to kilometres as in the case of earthquakes. Diverse problems involve multiscale approaches and are being addressed by current research: biotribology (e.g. eyes and joints lubrication), bioadhesion (e.g. how can flies and lizards walk on vertical walls?) and cell adhesion, rubber sliding and rolling friction, friction of the human skin, capillary bridges, adhesion in paper and in skin friction. All these problems involve surface roughness and interfacial surface interactions over many length scales and require an accurate multiscale contact mechanics theory. While several multi-scale approaches, e.g. based on fractal methods, have been proposed, at present the molecular-level physics and rheology and the large-scale phenomenological descriptions of friction are not connected satisfactorily. One of the current challenges of tribology is to bridge the different length scales and relate the emerging laws of friction to shorter and shorter length scales, down to the atomistic processes involved. In parallel, experimental methods need to proceed a step forward toward a more precise and faster control of the dynamics at the contact interface.
- Tuning nanofriction. Developing the ability to tune and manipulate frictional forces, adhesion and wear is a far reaching goal that can obviously be of high technological impact. Standard lubrication techniques used for macroscopic objects are less and less effective in the micro- and nano-world, because of dominant viscous and adhesive forces. Novel alternative solutions for the control and manipulation of friction at the microscopic scales are called for. Recent preliminary experimental and theoretical studies have, for example, successfully designed efficient methods and algorithms to control tribological properties by imposing tiny mechanical oscillations or by exploiting natural, or externally optimized, interfacial mating geometries. More ideas and approaches such as electrochemical manipulation of surface interactions to control nanofriction are currently under consideration.
- Confined lubricants under shear. One of the reasons making friction a complex task is the involvement of many degrees of freedom under a strict size confinement, which leaves very limited access to the sliding interface itself. Experimentally it is possible to shear nanometre thin lubrication films in the surface force apparatus (SFA). These films are also present in macroscopic systems so the SFA can predict processes such as wear by quantifying the squeeze-out of monolayers and friction by shearing of thin films. Theoretically, computer simulations such as Molecular Dynamics (MD) can simulate the tribological behaviour of very thin films. MD is becoming more and more sophisticated and the possibility of a direct visualisation of the sheared interface increases the probability of establishing robust models describing the tribological mechanisms in these films.
- Controlled nano movements. The manipulation of nano-objects is an emerging nanotribology subfield associating fundamental issues and high-risk technological goals. At present, despite the indisputable success of recent experimental studies, the conventional atomic force microscopy (AFM) and friction force microscopy (FFM) setups show severe inherent limitations (e.g., unsuitability to measure real contact area dependence, generally amorphous/disordered probe ends, limited availability of good-quality AFM tip materials). Indications are emerging that these difficulties can be overcome by manipulating well-defined nano-objects (adsorbed atomic islands or crystalline clusters) with the AFM setup because the interface under study is now the contact between a nano-object and the surface. Following this approach, the interfacial friction of prototype contacts of well-characterized size and structure has been measured recently. More realistic and technically relevant interfaces are waiting to be studied: achieving the controlled manipulation of these systems may open the possibility to build directly nano or molecular suprastructures and eventually to drive single nano-clusters or molecules on a surface.
With worldwide research on friction and wear focusing on molecular interactions at the nanoscale, the coordination provided by this Action will promote a synergy of expertise, to construct a gapless chain of knowledge connecting molecular forces and machinery lifetime, fundamental mechanisms of dissipation and energy consumption, toward a rational design of future technologies.